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Sterols may be found either as free sterols, acylated (sterol esters), alkylated (steryl alkyl ethers), sulfated (sterol sulfate), or linked to a glycoside moiety (steryl glycosides) which can be itself acylated (acylated sterol glycosides). 
Sterol biosynthesis is nearly ubiquitous among eukaryotes, it is almost completely absent in prokaryotes. They are not found in Archaea and the proven occurrences in bacteria are sparsely distributed and yield a limited array of products. The proteobacterium Methylococcus capsulatus and the planctomycete Gemmata obscuriglobus (Pearson A et al., PNAS 2003, 100, 15357) and some members of the myxobacteria are proven steroid-producing bacteria. As a result, the presence of diverse steranes (saturated 4-cycle skeleton) in ancient rocks is used as evidence for eukaryotic evolution 2.7 billion years ago. 

Structure of sterols with carbon numbers

Sterols are derived from the same squalene precursor as hopanoids but, in marked contrast, they are known to have an oxygen-dependent biosynthesis beginning with the formation of the first intermediate, 2,3- oxidosqualene. There is a close connection between modern-day biosynthesis of particular triterpenoid biomarkers and presence of molecular oxygen in the environment. Thus, the detection of steroid and triterpenoid hydrocarbons far back in Earth history has been used to infer the antiquity of oxygenic photosynthesis (Summons RE et al., Phil Trans R Soc B 2006, 361, 951). It has been hypothesized that increased levels of O2 in the atmosphere not only made the evolution of sterols possible, but that these sterols may in turn have facilitated the birth of complex organisms (the eukaryotes) (Chen LL et al., Biochem Biophys Res Comm 2007, 363, 885), likely in providing them with an early defense mechanism against O2 (Galea A et al., Free Rad Biol Med 2009, 47, 880; Brown AJ et al., Evolution 2010, April 14).


Sterols form an important group among the steroids.
Unsaturated steroids with most of the skeleton of cholestane containing a 3b-hydroxyl group and an aliphatic side chain of 8 or more carbon atoms attached to position 17 form the group of sterols. 


They are lipids resistant to saponification and are found in an appreciable quantity in all animal and vegetal tissues. Furthermore, cholestane may be considered as a biological marker compound valuable in the assessment of marine sediment maturity, even after hundreds of millions of years (Mackenzie AS et al., Science 1982, 217, 491). Sterols may include one or more of a variety of molecules belonging to 3-hydroxysteroids, they are C27-C30 crystalline alcohols (in Greek, stereos, solid). These lipids can be classed also as triterpenes, as they derive from squalene which gives directly by cyclization, unsaturation and 3b-hydroxylation, lanosterol in animals or cycloartenol in plants.

In the tissues of vertebrates, the main sterol is the C27 alcohol cholesterol (Greek, chole, bile), particularly abundant in adrenals (10%, w/w), nervous tissues (2%,w/w), liver (0.2%,w/w) and gall stones. The vertebrate brain is the most cholesterol-rich organ, containing roughly 25% of the total free cholesterol present in the whole body. Its fundamental carbon structure is a cyclopentanoperhydrophenanthrene ring (also called sterane). It was the first isolated sterol around 1758 by F.P. Poulletier de La Salle from gall stones. In 1815, it was isolated from the unsaponifiable fraction of animal fats by M.E. Chevreul who named it cholesterine (Greek, khole, bile and stereos, solid). The correct formula (C27H46O) was proposed in 1888 by F. Reinitzer but structural studies from 1900 to 1932, mainly by H.O. Wieland "on the constitution of the bile acids and related substances" (Nobel Prize Chemistry 1927) and by A.O.R. Windaus on "the constitution of sterols and their connection with the vitamins" (Nobel Prize Chemistry 1928), led to the exact steric representation of cholesterol. In 1936, Callow RK and Young FG have designated steroids all compounds chemically related to cholesterol. The main steps of cholesterol research have been reviewed up to the year 2000 (Vance DE et al., Biochim Biophys Acta 2000, 1529, 1). It must be noticed that the central role of cholesterol in atherogenesis has been proposed by a young Russian pathologist in Saint Petersburg in 1913 (Steinberg D, J Lipid Res 2013, 54, 2946).


Cholesterol is found in high concentrations in animal cell membranes, typical concentrations (expressed as molar percentage of total lipids) being about 30 mol%, ranging up to 50mol% in red blood cells and as high as 80 mol% in the ocular lens membranes (Li LK et al., J Lipid Res 1985, 26, 600). Consequently, cholesterol has numerous functions in membranes ranging from metabolism, as a precursor to hormones and vitamins, to providing mechanical strength and a control of the phase behavior of membranes (Rog T et al., Biochim Biophys Acta 2009, 1788, 97). It became clear that the key role of cholesterol  in the lateral organization of membranes and its free volume distribution seems to be involved in controlling membrane protein activity and "raft" formation (review in Barenholz Y, Prog Lipid Res 2002, 41, 1). At the cellular level, cholesterol may be replaced to some extent by some other sterols with minor modifications of the side chain (campesterol,
b-sitosterol) (Xu F et al., PNAS 2005, 102, 14551). Cholesterol is abundant in the femoral gland of the male lizard Acanthodactylus boskianus which uses it as a scent marking pheromone to establish dominance hierarchies (Khannoon ER et al., Chemoecology 2011, 21, 143).
In addition to these roles, cholesterol can form ester linkages with a class of secreted polypeptide signaling molecules encoded by the hedgehog gene family. These proteins function in several patterning events during metazoan development (Mann R et al., Biochim Biophys Acta 2000, 1529, 188).

Sponges, a primitive group of multicellular organisms (Poriphera), represent the richest source of bizarre sterols found in nature. Most sponges have the general sterol structure found in animals, plants, and fungi., i.e. cholesterol and sterols, but bearing one to three extra carbon atoms at C24. These side chains have been isolated with such unusual features as quaternary alkyl groups, cyclopropane and cyclopropene rings, allenes, and even acetylenes (Giner JL, Chem Rev 1993, 93, 1735).
24-Isopropylcholesterol is abundant and characteristic (with its analogue unsaturated at C22-C23) of the class Demospongiae. This sterol is absent in "true animals", the eumetazoans (cnidarians and bilaterian animals).


This demosponge sterane is abundant in sediment dating from the Neoproterozoic era (1,000-542 million years) and is the oldest evidence for animals in the fossil record (Love GD et al., Nature 2009, 457, 718).
Among the large list of sterols with cyclopropane ring, Nicasterol was identified in a Demospongiae, Calyx nicaensis.


While cholesterol was considered to be nearly absent in vegetal organisms, its presence is now largely accepted in higher plants. It can be detected in vegetal oils in a small proportion (up to 5% of the total sterols) but remains frequently present in trace amounts. An unusual relatively high content of cholesterol was described in camelina oil (about 200 mg per kg) (Shukla VKS et al., JAOCS 2002, 79, 965). However, several studies have revealed the existence of cholesterol as a major component sterol in chloroplasts, shoots and pollens. Furthermore, cholesterol has been detected as one of the major sterols in the surface lipids of higher plant leaves (rape) where he may amount to about 72% of the total sterols in that fraction (Noda M et al., Lipids 1988, 23, 439). Cholesterol is also dominant in most all Rhodophyceae algae, it is the only sterol presesnt in Laurencia paniculata (Al Easa H et al., Phytochemistry 1995, 39, 373).


In late-step synthesis of cholesterol, discrete oxidoreductive and/or demethylation reactions occur, which start with the common precursor lanosterol. Lanosterol is also found as a major constituent of the unsaponifiable portion of wool fat (lanoline) : about 15%. It has been shown that the bacterium (planctomycete), Gemmata obscuriglobus, is able to synthesized lanosterol and its uncommon isomer, parkeol (Pearson A et al., PNAS 2003, 100, 15357). No subsequent modifications of these sterols were observed. 
Several lanosterol derivatives have been identified in methanotrophic bacteria. The most abondant derivative, 4-methylcholestan-8(14),24-dien-3
b-ol, has been found first in Methylococcus capsulatus (Bouvier P et al., Biochem J 1976, 159, 267) and later in other similar bacteria.


Several compounds with the lanostane nucleus (ganoderates, lucidenates) have been isolated from a mushroom Polyporaceae (Ganoderma lucidum)
(Boh B et al., Biotechnol A